Markdown shortcuts

[cloverich]

Since migrating to plate (#78 ) the app uses more of a rich text editor style, although the persisted text is stored in markdown. Markdown support will eventually be truly first class. Here I want to add short-cuts so when you type certain markdown syntax, it auto-converts in the wysiwyg editor to the expected element

• using markdown headings should auto-convert to heading element
• using code (three backticks) should auto-convert to code block
• using italic, bold, and code markdown marks should auto-convert
• bullet lists should auto-convert as you type